1. Field of the Invention
This invention relates to a pneumatic radial tire requiring high running performance, and more particularly to a pneumatic radial tire capable of effectively realizing improvement of drainage property on wet road surface and reduction of pattern noise with excellent steering stability on dry road surface.
2. Related Art Statement
Among the conventional high running performance tires, tires mainly aiming at the improvement of drainage property are generally formed by arranging widthwise grooves each of which grooves continuously extends across a circumferential groove, in a region ranging from a central portion of a tread to a tread end. Particularly, these tires have a so-called unidirectional pattern that each of the widthwise grooves is inclined to approach its lower end portion viewed at a posture mounted onto a vehicle to the central portion of the tread toward the rotational direction of the tire. Moreover, the width of the widthwise groove is substantially uniform over a full length thereof, or is gradually enlarged toward the tread end.
In the high running performance tire aiming at the reduction of pattern noise, it is general to conduct a so-called phase shifting that portions of the widthwise groove separated through the circumferential groove are relatively displaced to each other in the circumferential direction of the tire.
When the widthwise groove is continuously extended across the circumferentail groove for improving the drainage property, it is preferable to make an inclination angle of the widthwise groove with respect to the meridional line of the tire large under such a condition that the width of the widthwise groove is, for example, equal over full length thereof for further improvement of the drainage property. Particularly, the inclination angle is in general to actually be within a range of 10.degree.-20.degree. in a shoulder portion of the tread from a viewpoint of a necessity for ensuring tire performances on dry road. As a result, in order to effectively improve the drainage property, there is no means other than the widening of the width of the widthwise groove. That is, the continuity of the widthwise groove in the widthwise direction of the tread and the widening of the width of the widthwise groove undesirably bring about an increase of pattern noise.
In order to reduce the pattern noise, the portions of the widthwise groove separated through the circumferential groove are relatively shifted to each other in the circumferential direction of the tire. In this case, however, it is obliged to degrade the drainage property on wet road surface.